1. Field of the Invention
The invention concerns variable-thickness sectors which are intended to form a disc for a rotary filter which may comprise a plurality of discs.
2. Discussion of Background
It has long been known for a continuous, liquid or gaseous phase and a dispersed, solid or liquid phase, initially forming a suspension resulting for example from an operation of attacking an ore, to be separated by filtration.
For that purpose, the suspension is passed over a support such as a grid, cloth, membrane, etc. on which the solid particles are deposited, forming a cake of variable thickness, while the liquid phase constituting the filtrate passes through the filtration texture and is then collected by any collecting means.
Depending on the characteristics of the suspension to be filtered such as for example their concentration of dry matter, the size of the solid materials, suitability for separation, aggressiveness of the medium and the degree of classification required, the man skilled in the art is required to choose a suitable piece of equipment, from the many technologies which are available on the market.
Among the filtration methods which are described in the specialist literature, one is used very frequently and considered to be of value in the field of separating liquid and solid phases of suspensions resulting from operations of attacking ores using hydrometallurgical processes: that filtration method involves using disc-type rotary filters such as those described for example in "Chemical Engineer's Handbook", page 978, by John H. PERRY, edition No 5, Graw Hill Book Company, 1950.
As described, the disc-type rotary filters comprise a plurality of discs which are fixed perpendicularly to a horizontal hollow shaft forming ducts in which various effects are produced such as suction, blowing etc. Each disc is formed by a plurality of independent sectors fitted with wire gauzes and/or textile filter bags, which are connected to the above-mentioned horizontal hollow shaft.
In the filtration operation, the discs provided with their sectors dip into a trough containing the suspension to be filtered, the solid phase being caused to cling to the filter cloths by suction while the separated liquid phase flows away in the collectors of the horizontal shaft which is provided with at least one distributor, by means of connections existing between each sector and the shaft itself. The solid phase which is disposed on the filter cloth is then drained off when it passes into the free air and then removed by blowing air in counter-flow relationship through the filter surfaces.
Now, it is well known that each rotary disc is formed by sectors, each of which consists of a rigid frame which imparts its shape to the sector, the frame containing the filter cloth support and a member for connection to the hollow collecting and drive shaft.
In accordance with the known art, the filtration cloth support is generally formed by performed metal materials such as for example flat or corrugated perforated sheet metal in order to have the minimum contact with the filter cloth. The two cloth supports which face is provided with a cloth support. The two cloth supports which are fitted into the frame of the sector are maintained at a constant spacing from each other by bracing members which fix the internal thickness of the sector, said thickness being defined by the amount of liquid phase to be discharged by way of the base of the sector, that is to say, close to the shaft, at the location where the sector is of smallest width. In other cases, the two filtration cloth supports and the bracing members which connect them may comprise for example a single corrugated sheet, the height of corrugation thereof constituting the internal thickness of the sector which is required for the discharge flow of liquid phase.
Thus, in the prior art, the thickness of a sector is constant from its periphery to the end which is in the vicinity of the shaft.
However, such constant-thickness sectors suffer from the major disadvantage of having a consequential internal volume which contains a substantial amount of the liquid phase which is discharged during the drainage step simultaneously with the fraction of liquid phase impregnating the solid phase deposited on the faces of the sector. However, the step of draining the liquid phase, which is fixed by the filtration step and the physical position of the sector at the moment of the blowing operation, is generally insufficient to permit complete removal of the liquid phase which is in the sector. Once the blowing step occurs, the fraction of the aqueous phase which is still in the sector is projected through the filtration cloths, re-wetting the solid phase which is still present on the cloth. That causes a reduction in the level of efficiency of the draining operation.
However, such constant-thickness sectors also suffer from another major disadvantage. In fact, even if the duration of the draining step is sufficient to remove from the sector the whole of the liquid phase which is initially present and which originates from the draining operation, it is often found that a part of the liquid phase is still present in the collector of the shaft at the moment at which the blowing operation is carried out. The presence of the liquid phase in that way causes a reduction in the section of the collector for the flow of air therethrough, thus increasing the pressure drops and accordingly reducing the level of efficiency of the blowing operation. It is true to say that such a disadvantage may be limited by increasing the diameter of the collector or by inclining the collector. However, those two options give rise to a fresh disadvantage which is an increase in the diameter of the collecting and driving shaft and all the equipment connected therewith, such as the distributor, bearings and sealing means for the shaft where it passes through the tank.
Faced with the above-mentioned disadvantages, the applicants, continuing their research, discovered and developed a sector for a disc-type filter, which reduces the internal volume of the sector while retaining a flow section which is sufficient, at the location where the width of the sector is at its smallest, that is to say in the vicinity of the shaft, to permit discharge of the liquid phase contained in the sector and the collector before the blowing operation is carried out.